Classification of nanomaterials
introduction to nanomaterials...
Nano technology-Introduction: The emerging fields of nanoscience and nanoengineering are leading to unprecedented understanding and control over the fundamental building blocks of all physical matter. This is likely to change the way almost everything —from vaccines to computers to automobile tires to objects not yet imagined —is designed and made. Some notes on scale 1A0=10-10m 1 nm=10-9m atomic/crystallographic 1 µm=10-6m micro structure
1mm=10-3m macro structure 1cm=10-2m The word ―Nano‖ means dwarf in Greek language. Use it as a prefix for any unit like a second or a meter and it means a billionth of that unit. A nanosecond is one billionth of a second. And a nanometer is one billionth of a meter—about the length of a few atoms lined up shoulder to shoulder. A world of things is built up from the tiny scale of nanometers. The thousands of cellular proteins and enzymes that constitute eg., The human bodies are a few nanometers thick. Enzymes typically are constructions of thousands of atoms in precise molecular structures that span some tens of nanometers. That kind of natural nanotechnology is about ten times smaller than some of the smallest synthetic nanotechnology that has been prepared until now. The individual components of an Intel Pentium III microprocessor span about 200 nanometers. This is the reason that computing is so powerful and easy these days. Nanotechnology makes microelectronics to be mere hints of what will come from engineering that begins on the even smaller scales of nanostructures.
Figure: The whole size issue; (a) Less than a nanometre- individual atoms are up to a few angstroms, or upto a few tenths of a nanometer in diameter; (b) NanometerTen shoulder-to-shoulder hydrogen atoms (blue balls) span 1 nm. DNA molecules are about 2.5 nm wide; (c) Thousands of nanometers- Biological cells, like these red blood cells, have diameters in the range of thousands of nm; (d) a million nanometers- A pinhead sized patch of this thumb (black point) is a million nanometers across; (e) Billions of nanometers-a two meter tall person is two billion nanometers tall.
Nanostructure science and technology is a broad and interdisciplinary area of research and development activity that has been growing explosively worldwide in the past few years. It has the potential for revolutionizing the ways in which materials and products are created and the range and nature of functionalities that can be accessed. It is already having a significant commercial impact, which will assuredly increase in the future.
Figure: Evolution of science & technology and the future. Emergence of Nano technology: Nanotechnology is new, but research on nanometer scale is not new at all.
The Chinese are known to use Au nano particles as in inorganic dye to introduce red color into their ceramic porcelains more than thousand years ago. A comprehensive study on the preparation and properties of colloidal gold was first published in the middle of the 19th century. Colloidal dispersion of gold prepared by Faraday in 1857 was stable for almost a century before being destroyed during world war-II.
Colloidal gold is used for treatment of arthiritis. In 1947 Dec 23 at AT&T Bell lab, the original Cm scale transistor made by Bardeen, Brattain, and Shockley at AT&T Bell lab. With the evolution of semiconductor industry, there is continuous decrease in device dimensions, today‘s transistors have well fallen in nanometer range. The discovery of synthetic materials, such as carbon fullerenes, carbon nano tubes and ordered mesomorphous materials has further fuelled the research in nanotechnology and nanomaterials. With the invention and development of scanning tunneling microscopy in the early 1980‘s and subsequently SPM, AFM, TEM, it is possible to study and manipulate the nanostructures and nanomaterials to a great detail and often down to the atomic level. Nanotechnology is not new, it is the combination of existing technologies and our new found ability to observe and manipulate at the atomic scale, this makes NT so compelling from scientific point. The following are the mile stones in the evolution of Nanotechnology. 3.5 Mrd.years: First cells with nano machines. 400 B.C: Demokrit: Reasoning about atoms and matter. 1905: Albert Einstein: Calculated molecular diameter. 1931: Max knoll&Ernst Ruska: Electron microscope 1959: Richard Feynman: There is plenty of room at the bottom. 1968: Alfred Y.Cho & JohnArthur (Bell Labs): MBE (atomic layer growth). 1974: Norio Taniguchi: Nanotechnology for fabrication methods below 1µm. 1981: Gerd Binnig& Heinrich Rohrer: Noble prize for inventing STM. 1985: Robert F.Carl, Harald W.Kroto: Richard smalley: Bucky balls. 1986: K.Eric Drexler: writing with a STM tool. 1991: Sumio Ligima: Carbon Nanotubes. 1993: Warren robinett, R.Stanley Williams: Combination of SEM and VR (virtual reality system). 1998: Cees Dekkar et all: Carbon nanotube transistor. 1999: James M.Tour& Mark.A.Read: Single molecule switch. 2000: Eigler et all: Construction of quantum mirrors. 2001: Florian Bambers: Soldering of nanotube with e-beam.
2004: Intel launches the Pentium iv ‗PRESCOFT‘ processor based on 90nm technology. “One nanometer is a magical point on the dimensional scale”. Why? One nanometer is a magical point on the dimensional scale. Because there is a sudden shift of all properties of material when they just enters into the nanoscale. As material size reduces from centimeter (bulk) to nanometer scale, properties mostly decreases as much as six orders of magnitude to that at macro level. The reason for this change is due to the nature of interactions among the atoms that are averaged out of existence in the bulk material. The same can be explained in another way i.e., surface energy increases with the overall surface area which in turn strongly dependent on the dimension of material. As nanostructures are having reduced dimensions, it leads to increase in surface energy via increase in surface area. The change in properties from macro scale to nano scale can be observed by taking a simple example as given below. Let us take an imaginary cube of gold 3 feet on each side. It is sliced in half along its length, width and height to produce eight little cubes, each 18 inches on a side. If we continue cutting the gold in this way from inches to centimeters, from centimeters to millimeters, and from millimeters to microns; we still notice no change in properties of gold between each stage except cash value and weight. All gold cubes are soft, shiny yellow and having same melting point. But when these µm size gold particles are further sliced into nano size particles, every thing will be changed including gold‘s color, melting point and chemical properties. Melting point of nano gold is less than that of bulk gold melting point. Similarly instead of yellow color, nano gold particles appear in different color. This color depends on the size of the particle. Not only for gold, all the materials will show the peculiar behavior and change in their properties when they enter into the nano scale. That is why one nanometer is called as a magical point on the dimensional scale. Nano technologyDefinition: BACK This is a term that has entered into the general and scientific vocabulary only recently but has been used at least as early as 1974 by Taniguchi. Nanotechnology is
defined as a technology where dimensions and tolerances are in the range of 0.1-100 nm (from size of the atom to about the wavelength of light) play a critical role. This definition is however too general to be of practical value because it could as well include, for example, topics as diverse as X-ray crystallography, atomic physics, microbial biology and include the whole of chemistry! The field covered down by nanotechnology is narrowed down to manipulation and machining within the defined dimensional range by technological means, as opposed to those used by craftsman, and thus excludes, for example, traditional glass polishing or glass colouring techniques. Another popular definition for Nano technology is : “Nano technology relates to the ability to build functional devices based on the controlled assembly of nano scale objects for specific technological applications.” Difference between Nano science and Nano technology: Study on fundamental relationships between physical properties and phenomena and material dimensions in the nanometer scale referred to as Nano science. But Nano technology is the application of these nano structures and principles behind them to make nano scale devices and to produce new materials. Feynman predictions on Nano technology: One of the first to advocate a future for nanotechnology was Richard Feynman, a Physics Nobel laureate who died in 1988. In late 1959 at the California Institute of Technology, he presented what has become one of 20th century science‘s classic lectures entitled ―There is Plenty of Room at the Bottom‖. This classic lecture has become part of the nanotechnology community‘s founding liturgy. Feynman got his motivation from biology since biological systems can be exceedingly small. He said, ―Many of the cells are very tiny, but they are active; they manufacture substances; they walk around; they wiggle; and they do all kind of marvelous things–all on a very small scale. Also, they store information. Consider the possibility that we too can make a thing very small which does what we want—that we can manufacture an object that manoeuvres at that level!‖ Feynman talked about nanotechnology before the word existed. Feynman dreamed with a technological vision of extreme miniaturization in 1959, several years before the word ―chip‖ became part of our every day life. Extrapolating from known physical laws, Feynman argued it was possible (with, say, an electron beam that could form lines in materials) to write all 25,000 pages of the 1959 edition of the Encyclopedia Britannica in an area
the size of a pin head! He calculated that a million such pinheads would amount to an area of about a 35 page pamphlet. Feynman further added ―All of the information which all of mankind has ever recorded in books can be carried in a pamphlet in your hand–and not written in code, but a simple reproduction of the original pictures, engravings and everything else on a small scale with-out loss of resolution.‖ And that‘s just how his talk began. He outlined how, with proper coding, all the world‘s books at the time actually could be stored in something the size of a dust speck, with each of the billions of bits in those books requiring a mere 100 atoms to store. How about building computers using wires, transistors, and other components that were that small? ―They could make judgments,‖ Feynman predicted. He discussed about using big tools to make smaller tools suitable for making yet smaller tools, and so on, until researchers had tools sized just right for directly manipulating atoms and molecules. Feynman further predicted that we will be able to literally place atoms one by one in exactly the arrangement that we want. ―Up to now,‖ he added, ―we have been content to dig in the ground to find minerals. We heat them and we do things on a large scale with them, and we hope to get a pure substance with just so much impurity, and so on. But we must always accept some atomic arrangement that nature gives us...I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do.‖ Repeatedly, during this famous lecture, Feynman reminded his audience that he wasn‘t joking. ―I am not inventing anti-gravity, which is possible someday only if the laws are not what we think,‖ he said. ―I am telling you what could be done if the laws are what we think; we are not doing it simply because we haven‘t yet gotten around to it.‖ Moore’s Laws: Gordon Moore, one of the founders of the Intel corporation, came up with two empirical laws to describe the amazing advances in integrated circuit electronics. Moore‘s first law (usually referred to simply Moore‘s law) says that the amount of space required to install a transistor on a chip shrinks by roughly half every 18 months. This means that the spot that could hold one transistor 15 years ago can hold 1000 transistors today. Moore‘s first law is good news. The bad news is Moore‘s second law. It is really a corollary to the first, which gloomily predicts that the cost of building a chip manufacturing plant (also called a
fabrication line or just fab) doubles with every other chip generation, or roughly every 36 months. The following figure shows Moore‘s laws in a graphical way. Role of Bottom-up and Top-Down approaches in Nano technology: BACK
Figure: Schematic representation of the building up of Nanostructures. There are two approaches for synthesis of nano materials and the fabrication of nano structures. Top down approach refers to slicing or successive cutting of a bulk material to get nano sized particle. Bottom up approach refers to the build up of a material from the bottom: atom by atom, molecule by molecule or cluster by cluster. Both approaches play very important role in modern industry and most likely in nano technology as well. There are advantages and disadvantages in both approaches.
Attrition or Milling is a typical top down method in making nano particles, where as the colloidal dispersion is a good example of bottom up approach in the synthesis of nano particles. The biggest problem with top down approach is the imperfection of surface structure and significant crystallographic damage to the processed patterns. These imperfections which in turn leads to extra challenges in the device design and fabrication. But this approach leads to the bulk production of nano material. Regardless of the defects produced by top down approach, they will continue to play an important role in the synthesis of nano structures. Though the bottom up approach oftenly referred in nanotechnology, it is not a newer concept. All the living beings in nature observe growth by this approach only and also it has been in industrial use for over a century. Examples include the production of salt and nitrate in chemical industry. Although the bottom up approach is nothing new, it plays an important role in the fabrication and processing of nano structures. There are several reasons for this and explained as below. When structures fall into a nanometer scale, there is a little chance for top down approach. All the tools we have possessed are too big to deal with such tiny subjects. Bottom up approach also promises a better chance to obtain nano structures with less defects, more homogeneous chemical composition. On the contrary, top down approach most likely introduces internal stress, in addition to surface defects and contaminations. Challenges in Nanotechnology: BACK Although many of the fundamentals have long been established in different fields such as in physics, chemistry, materials science and device science and technology, and research on nano technology is based on these established fundamentals and technologies, researchers in the field face many new challenges that are unique to nanostructures and nano materials. Challenges in nano technology include the integration of nano structures and nano materials into or with macroscopic systems that can interface with people. Challenges include the building and demonstration of novel tools to study at the nanometer level what is being manifested at the macro level. The small size and complexity of nanoscale structures make the development of new measurement technologies more challenging than ever. New measurement techniques need to be
developed at the nanometer scale and may require new innovations in metrological technology. Measurements of physical properties of nanomaterials require extremely sensitive instrumentation, while the noise level must be kept very low. Although material properties such as electrical conductivity, dielectric constant, tensile strength, are independent of dimensions and weight of the material in question, in practice, system properties are measured experimentally. For example, electrical conductance, capacitance and tensile stress are measured and used to calculate electrical conductivity, dielectric constant and tensile strength. As the dimensions of materials shrink from centimeter or millimeter scale to nanometer scale, the system properties would change accordingly, and mostly decrease with the reducing dimensions of the sample materials. Such a decrease can easily be as much as 6 orders of magnitude as sample size reduces from centimeter to nanometer scale. Other challenges arise in the nanometer scale, but are not found in the macro level. For example, doping in semiconductors has been a very well established process. However, random doping fluctuations become extremely important at nanometer scale, since the fluctuation of doping concentration would be no longer tolerable in the nanometer scale. With a typical doping concentration of 1018/ cm3, there will be just one dopant atom in a device of 10x10x10nm3 in size. Any distribution fluctuation of dopants will result in a totally different functionality of device in such a size range. Making the situation further complicated is the location of the dopant atoms. Surface atom would certainly behave differently from the centered atom. The challenge will be not only to achieve reproducible and uniform distribution of dopant atoms in the nanometer scale, but also to precisely control the location of dopant atoms. To meet such a challenge, the ability to monitor and manipulate the material processing in the atomic level is crucial. Furthermore, doping itself also imposes another challenge in nanotechnology, since the self purification of nanomaterials makes doping very difficult. One more challenge faced by researchers is ―all the mathematical models available for macro materials are not applicable to nanoscale materials. They must be developed to predict the behaviour of nano materials.” For the fabrication and processing of nanomaterials and nanostructures, the following challenges must be met: 1) Overcome the huge surface energy, a result of enormous surface area or large surface area to volume ratio.
2) Ensure all nanomaterials with desired size, uniform size distribution, morphology, crystallinity, chemical composition, and microstructure,that altogether result in desired physical properties. 3) Prevent nanomaterials and nanostructures from coarsening through either Ostwald ripening or agglomeration as time evolutes. Some present and future applications of nanomaterials: Here we list some of the present and future applications of nanomaterials that have been reported in recent literature: In electronics & optoelectronics:
‗nanophosphors‘ for affordable high-definition television and flat panel displays.
electroluminescent nanocrystalline silicon, opening the way for optoelectronic chips and possibly new type of color displays. efficient light-emitting diodes based on quantum dots with a voltage-controlled, tunable output color. powder or plastic layers using nanoparticles as an active scattering medium. optical switches and fibers based on nonlinear behavior. transparent conducting layers. three-dimensional optical memories.
materials for the next-generation computer chips. single-electron tunneling transistors using nanoparticles as quantum dots. efficient electrical contacts for semiconductor devices. electrically conducting nanoceramics. capacitive materials for, e.g., dynamic random access memories (DRAM).
magnetic memories based on materials with a high coercivity. magnetorestrictive materials, important for shielding components and devices. soft magnetic alloys such as Finemet resistors and varistors (voltage-dependent
resistors). high-temperature superconductors using nanoparticles for flux pinning.
graded refractive index (GRIN) optics: special plastic lenses. anti-fogging coating for spectacles and car windows. inexpensive colored glasses and optical filters.
In energy storage:
Novel solar cells, such as the Gratzel cell based on TiO2 materials.
window layers in classical solar cells utilizing the increased band gap due. high energy density (rechargeable) batteries. Smart windows based on the photochrome effect or electrical magnetic orientation effects. better thermal or electrical insulation materials, again using the higher gap. nanocrystalline hydrogen storage materials. Magnetic refrigerators from superparamagnetic materials.
In Gas sensing devices:
Gas sensors for Nox, Sox, CO, CO2, CH4 and aromatic hydrocarbons. UV sensors and robust optical sensors based on nanostructured silicon carbide (SiC). Smoke detectors. Ice detectors. Ice detectors on aircraft wings.
Cost-effective corrosion protection materials. Elimination of pollutants in catalytic converters utilising the large surface area of nanomaterials. Scratch-resistance top-coat using hybrid nanocomposite materials.
longer-lasting medical implants of biocompatible nanostructured ceramic and carbides. bio-compatible coating for medical applications. magnetic nanoparticles for hyperthermia. controlled drug release and drug delivery.
photocatalyst air and water purifiers. better activity, selectivity and lifetime in chemical transformations and fuel cells. precursors for a new type of catalyst (Cortex-catalysts). stereoselective catalysis using chiral modifiers on the surface of metal nanoparticles.
SYNTHESIS AND PROCESSING OF NANO POWDERS
1. Introduction Zero dimensional nano structures are the three dimensional relatively equi axed materials having their largest dimension in between ‗0‘ to ‗100‘ nm. These are also called as nano particles or nano powders. Nano particles have created a
high intrest in recent years by virtue of their unusual mechanical, electrical, optical and magnetic properties. Due to their special properties, nano particles are finding their wide applications in all fields of engineering, medicine like longer lasting medical implants of biocompatible nanostructured ceramic and carbides, biocompatible coating for medical applications, drug delivery, protection coatings, composite materials, anti fogging coatings for spectacles and car windows etc. Many techniques, including both top-down and bottomup approaches, have been developed and applied for the synthesis of nanoparticles. This chapter deals with some of the common as well as some of the more obscure processes that have been employed in recent years with an emphasis on production of nano particles.
Processes for producing ultrafine powders There are two approaches to the synthesis of nanomaterials and the fabrication of nanostructures; viz top-down and bottom-up. Top down approach involves the breaking down of the bulk material into nano sized structures orparticles. These techniques are an extension of those that have been used for producing micronsized particles. An example of such a technique is highenergy wet ball milling. The alternative approach, which has the potential of creating less waste and hence the more economical, is the ‗bottomup‘. Bottom up approach refers to the build up of a material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-bycluster. Many of these techniques are still under development or are just beginning to be used for commercial production o f nano powders.
Figure: Schematic representation of the ‗bottom up‘ and top down‘ synthesis processes of nanomaterials with the popular techniques that are used.
Characteristics of Nano particles that should posses by any fabrication technique: Getting merely a small size is not the only requirement. It should have i. Identical size of all particles (also called mono sized or with uniform size distribution. ii. Identical shape or morphology. iii. Identical chemical composition and crystal structure that are desired among different particles and within individual particles, such as core and composition must be the same. iv. Individually dispersed or mono dispersed i.e., no agglomeration. 2. Top down appraoach: BACK
High-Energy ball milling: The milling of materials is of prime interest in the mineral, ceramic processing, and powder metallurgy industry. Typical objectives of the milling process include particle size reduction (comminution), solid-state alloying, mixing or blending, and particle shape changes. These industrial processes are mostly restricted to relatively hard, brittle materials which fracture, deform, and cold weld during the milling operation. While oxide-dispersion strengthened super alloys have been the primary application of mechanical attrition, the technique has been extended to produce a variety of nonequilibrium structures including nanocrystalline, amorphous and quasicrystalline materials. A variety of ball mills has been developed for different purposes including tumbler mills, attrition mills, shaker mills, vibratory mills, planetary mills, etc. The basic process of mechanical attrition is illustrated in fig below.
Fig: Schematic representation of the principle of mechanical milling. Powders with typical particle diameters of about 50 µm are placed together with a number of hardened steel or tungsten carbide (WC) coated balls in a sealed container which is shaken or violently agitated. The most effective ratio for the ball to powder masses is five to 10. High-energy milling forces can be obtained using high frequencies and small amplitudes of vibration. Shaker mills (e.g. SPEX model 8000) which are preferable for small batches of powder (approximately 10 cm3 is sufficient for research purposes) are highly energetic and reactions can take place one order of magnitude faster than with other types of mill. Since the kinetic energy of the balls is a function of their mass and velocity, dense materials (steel or tungsten carbide) are preferable to ceramic balls. During the continuous severe
plastic deformation associated with mechanical attrition, a continuous refinement of the internal structure of the powder particles to nanometer scales occurs during high energy mechanical attrition. The temperature rise during this process is modest and is estimated to be less than or equal to 100 to 2000 C. The difficulty with top-down approaches is ensuring all the particles are broken down to the required particl e size. Furthermore, for all nanocrystalline materials prepared by a variety of different synthesis routes, surface and interface contamination is a major concern. In particular, during mechanical attrition, contamination by the milling tools (Fe) and atmosphere (trace elements of O2, N2, in rare gases) can be a problem. By minimizing the milling time and using the purest, most ductile metal powders available, a thin coating of the milling tools by the respective powder material can be obtained which reduces Fe contamination tremendously. Atmospheric contamination can be minimized or eliminated by sealing the vial with a flexible ‗O‘ ring after the powder has been loaded in an inert gas glove box. Small experimental ball mills can also be enclosed completely in an inert gas glove box. As a consequence, the contamination with Fe based wear debris can generally be reduced to less than 1-2 % and oxygen and nitrogen contamination to less than 300 ppm. However, milling of refractory metals in a shaker or planetary mill for extended periods of time (>30 h) can result in levels of Fe contamination of more than 10% if high vibrational or rotational frequencies are employed. On the other hand, contamination through the milling atmosphere can have a positive impact on the milling conditions if one wants to prepare metal or ceramic nanocomposites with one of the metallic elements being chemically highly reactive with the gas (or fluid) environment. On the other side, main advantage of top-down approach is high production rates of nano powders.